This invention relates to synchronous machines and, more particularly, to a switch for short-circuiting the main field rotor winding during induction start.
An AC voltage applied to the main stator winding of a synchronous machine, employed as a motor, causes the rotor to accelerate to synchronous speed. Acceleration of the rotor occurs as the AC voltage is impressed on the damper bars which are used as a squirrel cage during start-up. The AC voltage is also impressed upon the main field rotor winding. When the rotor is at stand-still, or slowly rotating, high voltages tend to be developed in the main field rotor winding. These voltages are not desirable and must be minimized during start-up to prevent damage to the electronic components and windings.
The prior art discloses a number of essentially resistant-free paths for short-circuiting the main field winding of a synchronous motor during start-up. These include bypass circuits for either short-circuiting the main field or for providing a resistive bypass during start-up. Most of the circuits for shorting the main field winding employ SCR's and zener diodes which provide a current bypass path during the start-up of the synchronous machine. Also, centrifugal switches have been employed to disconnect the main field rotor winding from the remainder of the circuit during start-up. These have been unreliable because they use mechanical linkages and contacts, and are relatively inaccessible.